Russia on Monday explained why fuel is being removed from a nuclear reactor in Iran, delaying plans to start up the giant plant this month. The Russian account, the first official rationale, came after experts last week offered theories of the failure ranging from a startup glitch to foul play.

In a statement, Russia’s state nuclear energy corporation, Rosatom, which is building the reactor in Bushehr, Iran, said it found damage to one of the reactor’s four main cooling pumps. That breakdown, it said, necessitated removal of the fuel core and an inspection of the reactor and its fuel assemblies to make sure they harbored no metal particles or chips. The detection of metal shards would prompt a thorough cleaning and, according to one official, a startup delay of up to two months.

Michael L. Corradini, a nuclear engineer at the University of Wisconsin, said that metallic debris whirling through a reactor’s cooling water posed no danger but that it would act as an abrasive that caused wear and tear, shortening the reactor’s life.

“It’s an operational issue,” he said in an interview. But while not suggesting an impact on safety, Dr. Corradini called the breakdown of the pump “not an ordinary occurrence at all.”

On Friday, the International Atomic Energy Agency said Iran told inspectors last Wednesday that it was planning to unload nuclear fuel from the reactor in Bushehr, on the Persian Gulf. Iran gave no explanation.

For years, Tehran has hailed the reactor as a showcase of its peaceful nuclear intentions and has pointed to its impending startup as a sign of quickening progress. Bushehr is Iran’s first nuclear power plant.

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The Russian explanation seems far more prosaic, although experts cautioned that the full capabilities of the Stuxnet virus remained unclear.

The Russian statement on Monday said the trouble arose as pressure mounted in the reactor during tests. The pump vibrated and joints broke, the statement said. As a result, metal shards smaller than three millimeters — or less than a tenth of an inch — could have shot into cooling pipes and lodged in fuel assemblies.

“The joints broke down under conditions of high vibration and pulsing pressure,” the statement said.

“If metal particles are found on the fuel assemblies,” it added, “they will be washed, the body of the reactor cleaned, and after this the fuel will again be loaded into the reactor.”

The statement said the failed pump dated to the 1970s, when West Germans began building the reactor. The Russians, who took over in 1995, have said for years that integrating the old German equipment posed more challenges than anticipated.

An article on Monday in Nezavisimaya Gazeta, a Moscow newspaper, cited an unidentified Russian official as saying that in the worst case, if metal shards were found in the fuel assemblies, the delay would amount to no more than two months.

Andrew E. Kramer contributed reporting from Moscow.

A version of this article appears in print on March 1, 2011, on Page A8 of the New York edition with the headline: Russians Say Damaged Cooling Pump Is Cause of Delay in Starting Iranian Reactor. Order Reprints|Today's Paper|Subscribe